This invention provides an optical power-limiting device in the LWIR region. Optical limiters are used to protect eyes, photodetectors, or cameras against unexpected strong illumination. An ideal optical limiter is transparent under low level of illumination at the wavelength of interest but “dark” under strong incident light power. The optical signal power transmitted through an optical limiter is ideally constant, no matter what the incident power is. An optical limiter should also have a low initiating threshold and a broad spectral range.
Interesting results in optical limiting have been obtained for Reverse Saturable Absorber (RSA) solutions and multi-photon absorber dyes. Such materials absorb more light as the intensity of the incident light increases. A disadvantage of such materials arises from their use of nonlinear optical absorption processes. This leads to a high threshold for the optical limiting behavior to switch on. More importantly, unlike the present invention, conventional RSA or multi-photon solutions cannot be used in the LWIR region.
U.S. Pat. No. 5,347,395 (“the '395 patent”) discloses an optical power limiting device whose transmissibility within a wave length range of interest is a function of the intensity of the incident radiation. The device utilizes detuning of the resonance of surface plasmons. The device of the '395 patent is said to be particularly suitable for the protection of IR detectors against incidence of high radiation intensities. However, the device of the '395 patent employs multiple layer interference, which has a relatively short bandwidth. The patent teaches in columns 4-5 that “the active layer” may be quartz, Al2O3, BeO, or AlN and that “the dispersion layer”, which is the index-changing material in the '395 patent, may be VO2.
In contrast to system described in the '395 patent, the device of the present invention works for a broad bandwidth, all the way from 7 to 14 microns. The device of the present invention is designed for use in continuous wave operation, such as sunlight, which is subject to relatively low peak intensity, unlike for instance lasers, which have very high peak intensities. The device of the '395 patent uses the signal itself to heat the index-changing material. In the present invention, the LWIR signal (7-14 μm wavelength) is the signal of interest and it is not absorbed by the index-changing material. Instead, in the present invention, the signal of interest is separated from radiation of other wavelengths in the incoming sunlight or other radiation. In this invention, visible and near-IR components of sunlight are utilized to heat the index-changing material.
A copending U.S. patent application Ser. No. 11/012,138 by Wu, Delakos, Lawrence, Lorraine, and Yenisch, entitled “Passive Broadband Long Wave and Mid-Wave Infrared Optical Limiter Device”, filed concurrently herewith, discloses an optical limiter that involves a transmission-type VO2-based thin film design. Since IR-transmitting materials usually have a high refractive index, Fresnel losses through the materials is higher than 10%, unless the materials are coated with Anti-Refraction material (“AR-coated”). The present invention provides a reflection type IR limiter design which can reflect up to 99% of the broadband radiation signal at normal state. The entire disclosure of this copending application is expressly incorporated herein by reference.